Video coding commonly uses two types of video frames: intra-frames, also known as key frames in a conventional context; and inter-frames. An intra-frame is compressed using only the current video frame, i.e. intra-frame prediction, similarly as in static image coding. An inter-frame on the other hand is compressed using the knowledge of one of the previously decoded frame, and allows for much more efficient compression when the scene has relatively little changes. Inter-frame coding is particularly efficient for, e.g., talking-head with static background, typical in video conferencing. Depending on the resolution, frame-rate, bit-rate and scene, an intra-frame can be up to 20-100 times larger than an inter-frame. On the other hand, an inter-frame imposes a dependency relation to previous inter-frames up to the most recent intra-frame. If any of those frames are missing, decoding the current inter-frame may result in errors and artifacts.
These techniques are used in, e.g., the H.264/AVC standard (see T. Wiegand, G. J. Sullivan, G. Bjontegaard, A. Luthra: “Overview of the H.264/AVC video coding standard,” in IEEE Transactions on Circuits and Systems for Video Technology, Volume: 13, Issue: 7, page(s): 560-576, July 2003).
Frequent and periodic transmission of intra-frames a.k.a. key frames is common in video streaming. This is illustrated schematically in FIG. 1, where key frames 1, 5, 9 and 13 etc. (shown black) are interleaved periodically between the transmission of inter-frames 2-4, 6-8, 10-12 and 14-16 etc. (shown white). The key frames are needed for two main reasons. Firstly, when a new user joins the session, he/she can only start decoding the video when a key-frame is received. Secondly, on packet loss, particularly bursty packet loss, the key-frame is a way to recover the lost coding state for proper decoding. The key frames allow the receiver to periodically update with “absolute” data, not relying on encoding relatively to previous frames, thus avoiding errors that could otherwise propagate due to packet loss occurs. However, the larger sizes of key frames incur a larger bandwidth for transmission. It would be desirable to try to mitigate this problem to some extent.